Nowadays, for cost-saving purposes, fluorescent lamp manufacturers use electronic ballasts of substantially the same circuit design. These electronic ballasts have been widely used in lamps configured for fluorescent light tubes and energy-saving light bulbs.
The basic power transmission structure of an electronic ballast typically consists of a half-bridge resonant circuit, either self-excited or separately excited in terms of control. FIGS. 1 and 2 show two self-excited oscillating circuits of commercially available energy-saving lamps, namely the 21W Edison energy-saving lamp made by GE of USA and the 23W HELIX energy-saving lamp by PHILIPS of the Netherlands. FIG. 3, on the other hand, shows a separately excited oscillating circuit designed by International Rectifier (IR) of USA for promoting its control IC IR2520D in energy-saving lamp applications (see IR's Application Note AN1066).
The working principles of the aforementioned electronic ballast circuits and their drawbacks are explained and analyzed as follows. Basically, the power transmission structure of all these electronic ballast circuits is a half-bridge inductor-capacitor (LC) resonant circuit including a pair of series-connected power switches (wherein Q1 is referred to as the upper-arm switch, and Q2 as the lower-arm switch), a direct-current (DC) blocking capacitor Cb, a resonant inductor Lr, a resonant capacitor Cr, and a control circuit for the power switches. The electronic ballast circuits in FIGS. 1 and 2 are self-excited circuits, which, generally speaking, must include a starter circuit. The starter circuit in FIGS. 1 and 2 is composed of R4, R6, C8, and DA1 (which is a diode for alternating current, or diac). The principle by which the self-excited circuits in FIGS. 1 and 2 can keep oscillating lies in a driving transformer which is made of an easily saturated magnetic core and configured for driving the power switches Q1, Q2 separately. The driving transformer includes three windings L20, L21, L22, wherein the primary winding L20 is series-connected in a resonant circuit, and the two secondary windings L21, L22 are connected, in opposite polarities, to the input ends of the power switches Q1, Q2 respectively. Once the lower-arm switch Q2 is turned on, the magnetic flux of the driving transformer varies between a positive saturation level and a negative saturation level, thereby turning on the power switches Q1, Q2 alternately. As a result, the resonant circuit, which consists of the resonant inductor Lr and the resonant capacitor Cr, begins to oscillate. Before the fluorescent light tube LAMP is lit, the light tube forms a substantially open circuit; therefore, the voltage at the resonant capacitor Cr keeps increasing until the argon gas in the fluorescent light tube LAMP is ionized, which in turn causes gasification and hence ionization of the mercury in the light tube. As the mercury ionization process generates ultraviolet light, the phosphor powder on the tube wall is excited by the ultraviolet light to emit visible light; thus, the fluorescent light tube LAMP is lit. At the same time, the gasified and ionized mercury causes a decrease in the voltage across the fluorescent light tube LAMP. Because of that, the light tube, when operating at a high frequency, functions as a resistor, and the circuit is turned into an oscillating inductor-resistor (LR) circuit, in which current intensity is determined by the saturation current of the driving transformer, and oscillation frequency is determined mainly by the resonant inductor Lr and the light tube resistor. As such, the fluorescent lamp remains lit.
The circuits in FIGS. 1 and 2 are different in that the light tube in FIG. 1 is connected in parallel to a resistor having a positive temperature coefficient, which resistor is generally known as PTC and has a resistance of several ohms under normal temperature. Immediately after the lamp is turned on, the filaments of the fluorescent light tube LAMP are preheated by the current flowing through the resistor PTC. Once the resistor PTC itself is heated by the current, the resistance of the resistor PTC rockets to the megohm level. As a result, the fluorescent light tube is lit by a high voltage generated by oscillation of the resonant capacitor Cr, while the preheated filaments emit hot electrons to ward off impact of the argon ions and thereby extend the service life of the filaments. However, should a mismatched resistor PTC be used, the argon gas may not be successfully ionized at the first time. As the argon gas is ionized repeatedly, the filaments are subject to repeated impact of the argon ions and may therefore end up having a shortened service life. This is probably the reason why the circuit in FIG. 2 dispenses with the resistor PTC having a positive temperature coefficient, and experiment results have justified such an omission.
The circuit shown in FIG. 3 is an oscillating circuit designed by International Rectifier to promote its control IC IR2520D in energy-saving lamp applications. In this separately excited circuit, the half-bridge power switches are both Power MOS, in which the upper-arm switch is indicated by MHS, and the lower-arm switch by MLS. In addition, LRES denotes a resonant inductor, CDC denotes a balancing capacitor, CRES denotes a resonant capacitor, and IR2520D denotes an upper-lower-arm half-bridge control circuit. The upper-lower-arm half-bridge control circuit sends out a signal whose frequency is, to begin with, higher than the resonant frequencies of the resonant inductor LRES and the resonant capacitor CRES and then gradually lowers. When the driving frequency is relatively high, resonant oscillation does not occur in the resonant inductor LRES or the resonant capacitor CRES; therefore, a current flows through the resonant capacitor CRES and the filaments to preheat the filaments. When the driving frequency falls to the vicinity of the resonant frequencies of the resonant inductor LRES and the resonant capacitor CRES, the resonant capacitor CRES begins resonant oscillation and thereby produces a high voltage to ionize the argon gas. After ignition, the driving frequency is fixed at a preset frequency to keep the light tube lit. The foregoing is the main operating principle of the circuit in FIG. 3.
Nevertheless, none of the three electronic ballasts described above has a preset self-protection mechanism to cope with the aging of a fluorescent light tube. As a fluorescent light tube ages, the voltage across the light tube tends to rise continuously, and the voltage may become so high that the related components and circuits will be burned. Moreover, with the light tube filaments in each of the electronic ballast circuits being connected in series between the resonant inductor and the resonant capacitor, energy on two sides of the resonant inductor and the resonant capacitor may begin a tug of war should any of the filaments suddenly break, either because of age of defects, thus causing sparks at and consequently an electric are between the disconnected ends of the broken filament. If the control circuit continues in operation, the high heat generated by the electric arc may burn up the broken filament, or even burn the post supporting that filament. In the latter scenario, the high temperature at the lamp head corresponding to the post may ignite the plastic components outside the lamp head and give rise to a fire accident. It should be noted that the electric arc will persist until the electronic ballast circuit is completely burned. Although the chances of creating an electric arc are only a few percent, the electric arc, once formed, is extremely dangerous and has serious consequences. Currently, a typical solution is to use relatively weak power switches so that the electronic ballast will burn prior to the light tube when the light tube becomes aged. While this expedient approach is very likely to pass the end-of-life tests of light tubes, the risks of fire accidents attributable to electronic ballasts remain if the power switches fail to burn before the light tubes as expected.
According to the above, the conventional electronic ballasts do not have a self-protection mechanism; therefore, when a light tube using such an electronic ballast reaches its natural end of life or a premature end of life, the electronic ballast will, by design, end its own life by burning its own circuit. As repair is cost-inefficient, there seems to be no better way to deal with a damaged energy-saving light bulb or electronic ballast than to discard it, which, however, is a waste of resources and causes severe pollution. Recently, with the increasing prevalence of environmental awareness, it has been a crucial issue for governments around the world and for all industries to recycle, repair, and reuse all sorts of articles, and yet the various electronic ballasts and fluorescent light tubes turn out to be a major obstacle, for they almost always end up with burned circuits—be they because of the age or defects of the light tubes—and are therefore totally unrecyclable. Hence, the major problem to be solved by the present invention is to design an electronic ballast which is structurally simple, self-protective, and completely recyclable. It is highly desirable that, when the filaments of a fluorescent light tube using this novel electronic ballast break because of age or defects, the self-protection mechanism of the electronic ballast not only can prevent the occurrence of electric arcs and fire accidents otherwise attributable thereto, but also can stop the operation of the resonant circuit in the electronic ballast immediately so that the resonant circuit is protected from excessive power consumption which may otherwise burn the related circuits and components in the electronic ballast.